Medulla Oblongata

Medulla oblongata

Medulla oblongata  Lower

part of brainstem.  Contains central canal in the lower part and its upper part contains IV ventricle.  Presents pyramids on either side of the midline anteriorly (formed by corticospinal tracts on their way to spinal cord) and olive produced by underlying inferior olivary nucleus.

Gross features On

the back of medulla are gracile and cuneate tubercles which contain nuclei of the same name(2nd order neurons) for the fasciculi of the same name.

Fasciculus gracilis and F.cuneatus  F.gracilis

(tract of Goll) is the medial of the two. Carries fine touch, vibration and conscious proprioception from the lower part of the body.  F. cuneatus (tract of Burdach) is lateral to the above tract and carries same sensations from the upper part of the body.

Tabes dorsalis  Tertiary

syphilitic degeneration of posterior white columns which only sensory tracts.  There is loss of tactile discrimination, vibration and position sense.

structure  The

characteristic appearance of grey matter in spinal cord is lost in medulla.  The grey matter contains the nuclei of last four cranial nerves and reticular formation.  For the arrangement of grey and white matters medulla is studied at 3 levels.

Medulla at the level of pyramidal decussation  

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Important features are: 1.Spinal nucleus and tract of V in line with the posterior horn of spinal cord. 2.Pyramidal decussation(75%of fibers cross the midline and descend to the cord as lat.corticospinal tracts.

continued  3.corresponding

to the anterior horn is the spinal nucleus of accessory.  4.diffuse zone of nerve cells and fibers corresponding to the lateral white column called the reticular formation.

The reticular formation

SPECIAL FEATURES OF V nerve  Has

a large sensory nucleus (V1,V2 and V3) and a smaller motor nucleus.  Hence it extends throughout the brain stem (mesencephalic nucleus, principal nucleus and spinal tract and nucleus).  Perhaps the only cranial nerve nucleus to do so.

Medulla at the level of sensory decussation 

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Fasciculi gracilis and cuneatus end in corresponding nuclei. Second order neurons begin as internal arcuate fibres,cross the midline and form the medial lemniscus. Olivary nucleus makes its appearance. Pyramids on either side of midline.

Sensory decussation

continued  Central

grey matter (close to the central canal) contains  1.hypoglossal nucleus (close to the midline)  2.dorsal nucleus of vagus (parasympathetic)  3.nucleus of tractus solitarius (taste)

Purely motor cranial nerves  3rd,4th,6th

and 12th cranial nerves lie in the same line and have their nuclei close to the midline of the brain stem.

continued  4.MLF

appears just posterior to medial lemniscus (this is connected to 3rd,4th,6th,8th and spinal nucleus of XI)  Spinocerebellar and lateral spinothalamic tracts move laterally to the lateral white column.  Lateral and anterior spinothalamic tracts lie close to each other and form “spinal lemniscus”.

Upper medulla (open part)  central

canal opens into IV ventricle.  along with XII, vestibular nuclei(2 of the 4) appear here.  nucleus ambiguus (common motor nucleus for 9th,10th and 11th cranial nerves) is present.

Medulla (clinical aspects)

Cranial nerves IX, X, XI and XII originate from the medulla. Functions of the medulla include: 1) control of somatic motor reflexes of the larynx, pharynx and tongue which are important in speech and swallowing. (nucleus ambiguus) 2) control of visceral motor reflexes, e.g., coughing, swallowing, salivating, vomiting, secretion, sneezing; the medulla also receives taste and other visceral sensations.

 3)

the MLF contains fibers for the regulation of the position of the head and neck and for coordinated eye movements;  4) the medulla is a relay for the cochlear and vestibular responses;  5) the reticular formation regulates a variety of functions including skeletal motor activity, consciousness, visceral functions, conduction of sensation, etc.

The features of medullary syndromes reflect its anatomy and physiology, i.e., the ascending sensory tracts, the descending motor systems, cranial nerves IX - XI exiting at this level, and the cranial nerves that are associated at or near this level, viz., CN VIII

Lateral medullary syndrome (PICA) of Wallenberg  Dorsolateral

aspect of medulla is supplied by PICA.  In this area spinothalamic tract, spinal nucleus and tract of V, nucleus ambiguus, inferior cerebellar peduncle, vestibular nuclei are situated. Effects

Inferior cerebellar peduncle

Ipsilateral ataxia; also vertigo from interruption of connections of vestibular nuclei

Spinal tract of trigeminal

Loss of pain and temperature sensation, same side of face

Spinothalamic tract

Loss of pain and temperature sensation, opposite side of the body

Nucleus ambiguus

Paralysis of vocal cord and pharyngeal muscles on the side of the lesion

Medial medullary syndrome (hypoglossal alternating hemiplegia)  Due

to thrombosis of vertebral artery branches supply paramedian regions of medulla.  Structures present here are: pyramid, hypoglossal nerve, medial lemniscus. Effects

pyramid

Contralateral hemiplegia

Hypoglossal Ipsilateral paralysis and nucleus atrophy of half of tongue (when protruded, the tip deviates to the same side of lesion) Medial lemniscus

Contralateral loss of position and vibration sense